Fluorosurfactants are typically used as a polymerization aid in the dispersion polymerization of fluoropolymers, the fluorosurfactants functioning as a non-telogenic dispersing agent. For example, an early description of this use of fluorosurfactants is found in U.S. Pat. No. 2,559,752 (Berry). Because of environmental concerns and because fluorosurfactants are expensive, processes have been developed for their recovery from waste water and from aqueous fluoropolymer dispersions.
One method for removal of fluorosurfactants from fluoropolymer dispersions is disclosed in U.S. Pat. No. 4,369,266 (Kuhls et al.) and includes the addition of a stabilizing surfactant followed by concentration by ultrafiltration. This patent teaches that a high proportion of the fluorosurfactant can be removed via the aqueous permeate. It is also known to remove fluorosurfactant by adsorption onto an ion exchange resin as taught in U.S. Pat. No. 3,882,153 (Seki et al) and U.S. Pat. No. 4,282,162 (Kuhls). Kuhls teaches recovery of fluorinated emulsifiers dissolved in the aqueous phase after coagulation of the polymer from the dispersion or in aqueous polymer dispersions to be concentrated. US 2003/0125421 A1 (Bladel et al.) also teaches removal of fluorine-containing emulsifiers from fluoropolymer dispersion by contacting with an anion exchanger.
Known processes using anion exchange resins for the removal of fluorosurfactant from fluoropolymer dispersions employ either strongly basic anion exchange resins or weakly basic anion exchange resins. Although weakly basic resins are useful because they can be regenerated more easily, strongly basis resins are preferred when it is desired to reduce fluorosurfactant to very low levels and for high utilization of the resin. Strongly basic ion exchange resins also have the advantage of less sensitivity to the pH of the media.
For practical commercial manufacture of fluoropolymer dispersion with low fluorosurfactant content, it is known to pass fluoropolymer dispersion through a fixed bed, such as a column of the anion exchange resin. However there is need to produce a uniform dispersion product, i.e., dispersion with a standard low level of reduced surfactant. Fluorosurfactant concentration analysis is routinely performed offline and takes around eight hours to complete the determination. Such a time lapse is not a practical way of determining whether acceptable product is being produced. Unacceptable product having a high fluorosurfactant content may result when a fixed bed is saturated such that the ion exchange sites are no longer adequately absorbing sufficient fluorosurfactant and thereby producing a dispersion with high, unacceptable fluorosurfactant content.
In a fixed bed such as a column, there is a working zone (mass transfer zone) that moves progressively through the column in the direction of flow of the dispersion such that the fluorosurfactant is absorbed onto the ion exchange resin in this working zone. Specifically, fluoropolymer dispersion enters the working zone having an initial fluorosurfactant content and exits the zone with reduced fluorosurfactant content. The fluoropolymer dispersion with reduced fluorosurfactant content passes through the remainder of the unsaturated column and exits the column. The working zone moves progressively through the column until the column is fully saturated. The point at which the column is fully saturated such that working zone has moved entirely though the column and there is no longer adequate absorption by the ion exchange resin is called breakthrough.
What is desired is a process for reducing the fluorosurfactant content of a stabilized fluoropolymer dispersion to detect working zone breakthrough at the end of a column, i.e., column saturation.